1. Field of the Invention
The present invention relates to novel tetraphenyldithiolene complexes, asymmetrically substituted benzoins as intermediates for their preparation, and an optical recording material which contains the novel tetraphenyldithiolene complexes.
2. Discussion of the Background
Recording materials in which radiation having a high energy density, eg. laser light, produces a localized change of state are known. This thermally produced change of state, for example vaporization, change in flow behavior or fading, is associated with a change in the optical properties, for example in the absorption as a result of a change in the absorption maximum, or in the extinction, and this change can be utilized for recording information or data.
Because of the small size of the element, its low energy demand and the possibility of direct modulation of the optical output power by modulating the electric driving current, solid state injection lasers which emit in the near infrared, especially the AlGaAs laser, which operates in the wavelength range from about 750 to 950 nm, are particularly useful as a light source for an optical recording system.
A large number of inorganic and organic materials are known for this application. These materials have an adequate absorption in this wavelength range and alter their optical properties by absorbing the radiation and hence the energy present therein in the stated wavelength range, by disintegration, vaporization, melting or in another manner.
The known information recording materials consist of a base on which thin layers of inorganic materials, eg. metals, semimetals, alloys or chalcogen glass, or organic compounds, such as IR dyes, are applied. The thin layers are produced, in particular, by vapor deposition under reduced pressure or by atomization techniques. The thickness of the layers should be chosen so that the total incident radiation is absorbed, unless it is intended to utilize intereference phenomena. The base can consist of glass or of suitable plastics, eg. a polycarbonate, polymethylmethacrylate, polystyrene, a polystyrene copolymer, polyvinyl chloride or polymethylpentene.
When used as a storage material, the amorphous layers must remain unchanged over prolonged periods.
Aging processes, eg. crystallization or fading due to light and heat, which alter the morphology of the storage layer take place relatively frequently in thin layers produced by vapor deposition. Neutral IR dyes in polymer films, ionic IR dyes in the form of lakes or IR chromophores chemically bonded to polymers should be more stable over prolonged periods. Moreover, the latter have the advantage that they can be prepared by a more economical process.
A large number of bases, reflective materials and laser light-sensitive layers for optical recording materials for use with semiconductor injection lasers are known. Organic IR dyes described are, in particular, phthalocyanine compounds, methine dyes and quadratic acid derivatives. Complex azo dyes, anthraquinone dyes and triphenylmethane dyes as well as pyrylium and thiopyrylium salts have likewise been described but only have a reStricted use since their absorption maximum in most cases is at too short a wavelength for the conventional semiconductor lasers. This problem does not arise in the case of the dithiolene complexes, a large number of which are known [for example, J. A. McCLEVERTY, Progr. Inorg. Chem. 10, (1968), 49-221; G. N. SCHRAUZER, Acc. Chem. Res. 2, (1969), 72-80] and whose use in optical recording materials has been described.
WO No. 83/02 428 describes an optical recording material which contains nickel-benzodithiolene complexes of the formula ##STR3## where X is hydrogen, 1-methyl, 1,4-dimethyl, 1,2,3,4-tetramethyl, 1-chloro or 1,2,3,4-tetrachloro and A is a tetraalkylammonium cation. These complexes absorb between 800 and 950 nm (.epsilon. less than 17,000).
DE-A No. 2 951 341 descloses a recording medium which can be writren on by means of an AlGaAs laser and carries, on a glass substrate, a light-reflecting layer, eg. gold, and a light-absorbing layer. The layers are produced on the substrate by vapor deposition under reduced pressure. The light-absorbing layer consists of dithiolene complexes of the formula ##STR4## where R is unsubstituted or substituted phenyl.
A similar system is described in U.S. Pat. No. 4,320,489 and takes the form of a reversible optical recording medium which possesses on the surface, as a photosensitive layer, a thermoplastic layer which contains compounds of the formula ##STR5## where Me is nickel, palladium or platinum and R' and R" are each alkyl or unsubstituted or substituted phenyl. However, no information is given regarding the quality of the recording medium, for example in the form of read or write energies or signal-to-noise ratios.
When other IR dyes of the formula (III) which are disclosed in the literature were used, it was found that these IR dyes are not sufficiently soluble in the polymers used, so that the recording media produced with them, although capable of being written on, were of unsatisfactory quality.
Our own experiments have wshown that all known dithiolene complexes and the other, very pure dithiolene complexes stated above and mixtures of these compounds, which can be deposited as a thin layer on a base by vapor deposition under reduced pressure, crystallized more or less rapidly after vapor deposition or after further exposure to heat, with the result that these dye layers are useless as a material for an optical recording medium. In addition to the known disadvantages of batchwise vaporization under reduced pressure during mass production, recording media of this type are very unlikely to possess long-term stability.
Res. Discl. 21 612/1982 discloses dithiolene complexes of the formula (III), where the radicals R' independently of one another are each aryl, such as alkylaryl or alkoxyaryl, R" is hydrogen or unsubstituted or substituted alkyl and Me is nickel. These complexes absorb between about 700 and 900 nm and possess good solubilities in various organic solvents.